The present invention relates to a three-phase combined type gas insulated electrical apparatus comprising a sealed metal casing in which electrical conductors for three phases are collectively arranged.
Generally, a gas insulated electrical apparatus comprises a metal cylinder-shaped casing filled with an insulating gas such as SF.sub.6 gas and high voltage electrical conductors arranged within the casing along the axis thereof. A gas insulated circuit breaker or a gas insulated bus is known as such a type of apparatus.
The diameter of the cylinder-shaped casing of this type of apparatus is determined by the distance of electrical insulation between the high voltage electrical conductor and the cylinder-shaped casing. Another factor which must be taken into consideration in determining the diameter of the cylinder-shaped casing is the fact that the increase of the temperature of the casing is limited in many cases. As an example, a non-magnetic material is used for part of the circumferential portion of the cylinder-shaped casing. Japanese Patent Publication No. 40576/75, for instance, discloses an apparatus in which a non-magnetic material such as aluminum is used as part of the cylinder-shaped iron casing in order to reduce the iron loss due to the magnetic field caused by the current flowing in the high voltage electric conductors, thus preventing the casing from increasing in temperature.
If a three-phase combined type gas insulated bus of 500 KV in rated voltage and 8000 A in rated current is fabricated according to the specification for application of this art, however, an outer diameter of the cylinder-shaped casing is enlarged to such an extent as about 1800 mm. This figure does not make use of the advantage that the use of SF.sub.6 gas or the like superior in insulating characteristics shortens the insulation distance.